This invention relates to corrosion-resistant sight or inspection glasses for pressure vessels.
Alkaline aqueous solutions are frequently utilized in pressure vessels at temperatures of up to 300.degree. C., but the sight glasses incorporated in a conventional manner in the side of such pressure vessels generally do not exhibit adequate resistance to deterioration, irrespective of whether the glasses are hard or soft glasses, tempered of not. As a consequence, to avoid failure of the sight glass during operation of such a vessel, the sight glasses must be exchanged frequently, which is expensive.
As an attempt to remedy the problem, mica plates have been placed in front of the sight glasses to protect the latter. Whereas, this results in an improvement since mica is more stable with respect to alkalis than the glasses employed, protection is limited and depends on the quality of the micas. Additionally, on account of the diminishing availability of optical grade micas, the cost thereof has risen dramatically in recent times and has even exceeded the price of the sight glasses to be protected. Consequently, the use of mica will become a prohibitively expensive solution to the problem of failure of sight glasses, particularly since the availability of optical grade mica is expected to become worse in the future.
Another attempted solution has been to coat the sight glasses with a synthetic resin on at least the interior side, i.e., the side of the glass in contact with the fluid inside the pressure vessel. One drawback of this expedient is that the temperature stability of the plastics employed is unsatisfactory above 200.degree. C. whereas boilers are frequently operated up to 300.degree. C. Even below 200.degree. C., damage to the synthetic resin and resultant separation from the glass substrate occur due to steam diffusion and erosion from the turbulent conditions at the boiler window. In all these cases, the entire sight glass must be replaced.
For these reasons, another suggested solution to the problem is based on providing the inspection glass with a transparent synthetic resin sheet resistant to aggressive, especially alkaline, solutions. This sheet is merely fixed in place mechanically and thus can be exchanged without replacing the glass as well. For this purpose, transparent fluoropolymers have been suggested, especially polytetrafluoroethylenehexafluoropropylene copolymers according to Austrian Pat. No. 329,790. These polymers are stable against typical, alkaline boiler feed waters containing phosphate and carbonate ions at a pH of 10 and at up to 200.degree. C. But here again, there is the disadvantage that the stability is limited to 200.degree. C. Another disadvantage is that steam diffuses through the sheet into the space between the sheet and the sight glass and hampers visibility. Also, the cost of such perfluoropolymers is considerable and, in conjunction with the aforementioned drawbacks, this solution to the problem, for all practical purposes, is restricted to a few special cases.